1. 7-1 Chapter 7 Structuring System Process Requirements Modern Systems Analysis and Design Fourth Edition

2. 7-2 Learning Objectives Understand logical process modeling via data flow diagrams (DFDs). Draw DFDs of well structured process models. Decompose DFDs into lower-level diagrams. Balance high-level and low-level DFDs. Explain differences between current physical, current logical, new physical, and new logical DFDs. Use DFDs for analyzing information systems.

3. 7-3

4. 7-4 Process Modeling A technique for graphically representing the processes that are used to capture, manipulate, store, and distribute data –between a system and its environment –among system components Build a DFD using information gathered during requirements gathering and determination Both processes and data structures are modeled in DFDs

5. 7-5 Process Modeling (cont.) Deliverables and Outcomes Context data flow diagram (DFD) –Shows the scope of a system (i.e., a top-level view) Often DFDs are created showing the current physical and logical system –It enables analysts to understand how the current system operates DFDs of new logical system –The DFD is independent of technology –It shows data flows, structure, and functional requirements of the new system Includes a thorough description of each DFD component

6. 7-6 Data Flow Diagram (DFD) A picture of the movement of data between external entities and the processes and data stores within a system How does a DFD differ from a systems flowchart? –DFDs depict logical data flow independent of technology –The focus is on data flows, not process flows alone

7. 7-7 DFD Symbols

8. 7-8 DFD Symbols (cont.) Process: work or actions performed on data (inside the system) Data store: data at rest (inside the system) Source/sink: external entity that is origin or destination of data (outside the system) Data flow: arrows depicting movement of data

9. 7-9 DFD Diagramming Rules Process No process can have only outputs or only inputs…processes must have both outputs and inputs. Process labels should be verb phrases.

10. 7-10 DFD Diagramming Rules Data Store Data store labels should be noun phrases. All flows to or from a data store must move through a process.

11. 7-11 DFD Diagramming Rules Source/Sink Source and sink labels should be noun phrases. No data moves directly between external entities without going through a process. Interactions between external entities without intervening processes are outside the system and therefore not represented in the DFD.

12. 7-12 DFD Diagramming Rules Data Flow Bidirectional flow between process and data store is represented by two separate arrows. Forked data flow must refer to exact same data item (not different data items) from a common location to multiple destinations.

13. 7-13 DFD Diagramming Rules Data Flow (cont.) Joined data flow must refer to exact same data item (not different data items) from multiple sources to a common location. Data flow cannot go directly from a process to itself, must go through intervening processes.

14. 7-14 DFD Diagramming Rules Data Flow (cont.) Data flow from a process to a data store means update (insert, delete or change). Data flow from a data store to a process means retrieve or use Data flow labels should be noun phrases.

15. 7-15 Functional Decomposition An iterative process of breaking a system description down into finer and finer detail High-level processes described in terms of lower-level sub-processes DFD charts created for each level of detail

16. 7-16 DFD Levels Context DFD –Overview of the organizational system Level-0 DFD –Representation of system’s major processes at high level of abstraction Level-1 DFD –Results from decomposition of Level 0 diagram Level-n DFD –Results from decomposition of Level n-1 diagram

17. 7-17 Context Diagram Context diagram shows the system boundaries, external entities that interact with the system, and major information flows between entities and the system. NOTE: only one process symbol, and no data stores shown.

18. 7-18 Level-0 DFD Level-0 DFD shows the system’s major processes, data flows, and data stores at a high level of abstraction. Processes are labeled 1.0, 2.0, etc. These will be decomposed into more primitive (lower-level) DFDs.

19. 7-19 Level-1 DFD Level-1 DFD shows the sub- processes of one of the processes in the Level-0 DFD. This is a Level-1 DFD for Process 4.0. Processes are labeled 4.1, 4.2, etc. These can be further decomposed in more primitive (lower-level) DFDs if necessary.

20. 7-20 Level-n DFD Level-n DFD shows the sub-processes of one of the processes in the Level n-1 DFD. This is a Level-2 DFD for Process 4.3. Processes are labeled 4.3.1, 4.3.2, etc. If this is the lowest level of the hierarchy, it is called a primitive DFD.

21. 7-21 DFD Balancing The conservation of inputs and outputs to a data flow process when that process is decomposed to a lower level Balanced means: –Number of inputs to lower level DFD equals number of inputs to associated process of higher-level DFD –Number of outputs to lower level DFD equals number of outputs to associated process of higher-level DFD

22. 7-22 Unbalanced DFD This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs. 1 input 1 output 2 inputs 1 output

23. 7-23 Balanced DFD These are balanced because the numbers of inputs and outputs of context diagram process equal the number of inputs and outputs of Level-0 diagram. 3 outputs 1 input

24. 7-24 Balanced DFD (cont.) These are balanced because the numbers of inputs and outputs to Process 1.0 of the Level-0 diagram equals the number of inputs and outputs to the Level-1 diagram. 1 input 4 outputs

25. 7-25 Data Flow Splitting A composite data flow at a higher level may be split if different parts go to different processes in the lower level DFD. This remains balanced because the same data is involved, but split into two parts.

26. 7-26 More DFD Rules

27. 7-27 Four Different Types of DFD Current Physical –Process labels identify technology (people or systems) used to process the data. –Data flows and data stores identify actual name of the physical media. Current Logical –Physical aspects of system are removed as much as possible. –Current system is reduced to data and processes that transform them.

28. 7-28 Four Different Types of DFD (cont.) New Logical –Includes additional functions –Obsolete functions are removed –Inefficient data flows are reorganized New Physical –Represents the physical implementation of the new system

29. 7-29 Guidelines for Drawing DFDs Completeness –DFD must include all components necessary for system. –Each component must be fully described in the project dictionary or CASE repository. Consistency –The extent to which information contained on one level of a set of nested DFDs is also included on other levels.

30. 7-30 Guidelines for Drawing DFDs Timing –Time is not represented well on DFDs. –Best to draw DFDs as if the system has never started and will never stop. Iterative Development –Analyst should expect to redraw diagram several times before reaching the closest approximation to the system being modeled.

31. 7-31 Guidelines for Drawing DFDs Primitive DFDs –Lowest logical level of decomposition –Decision has to be made when to stop decomposition

32. 7-32 Guidelines for Drawing DFDs Rules for stopping decomposition –When each process has been reduced to a single decision, calculation or database operation –When each data store represents data about a single entity –When the system user does not care to see any more detail

33. 7-33 Guidelines for Drawing DFDs Rules for stopping decomposition (continued) –When every data flow does not need to be split further to show that data are handled in various ways –When you believe that you have shown each business form or transaction, online display and report as a single data flow –When you believe that there is a separate process for each choice on all lowest-level menu options

34. 7-34 Using DFDs as Analysis Tools Gap Analysis –The process of discovering discrepancies between two or more sets of data flow diagrams or discrepancies within a single DFD Inefficiencies in a system can often be identified through DFDs.

35. 7-35 Using DFDs in Business Process Reengineering Before: Credit approval process required six days After: process 100 times as many transactions in the same time

36. 7-36 Summary In this chapter you learned how to: Understand logical process modeling via data flow diagrams (DFDs). Draw DFDs of well structured process models. Decompose DFDs into lower-level diagrams. Balance high-level and low-level DFDs. Explain differences between current physical, current logical, new physical, and new logical DFDs. Use DFDs for analyzing information systems.